Mesovortices in Hurricane Michael
By now, most people have heard that Hurricane Michael was recently upgraded from Category 4 to 5 at landfall. I tuned in to a webinar recently where NHC Hurricane Specialists Stacy Stewart and Jack Beven explained the troves of data (radar, satellite, and aircraft) used in this decision. The webinar should be posted online here soon:
One of the interesting things about this case that Stacy Stewart mentioned was the fact that the decision on what wind data to use was complicated by the presence of mesovortices in the eyewall. Now, if you're not a hurricane person, you may not have heard of these. Basically, they're small-scale cyclones (mesocyclones) in the hurricane eyewall. A little bigger than a tornado, but much smaller than the scale of the storm itself. They make the eyewall take on shapes like a triangle, or pentagon, or other shapes rather than the typical smooth, circular eyewall we see. Here are a couple of radar images from during our last flight into Michael, illustrating these unusual shapes.
The mesovortices can cause small-scale local increases in wind speed. This makes it hard to tell if the wind being measured by the plane/radar is representative of the hurricane circulation as a whole, or just the small-scale mesovortex (which typically don't last very long). Now, as Forecaster Stewart mentioned, the difference doesn't matter to the people being hit by it (although they determined the winds were Cat 5 with or without these mesovortices, and there weren't as many in Michael by the time it made landfall). But what's interesting to me is I was able to replicate this structure in a simulation of Hurricane Michael using a test model (called HAFS, the Hurricane Analysis and Forecast System) that is a high-resolution nested version of the FV3GFS model.
Here is a forecast for Michael at landfall (started using data from October 7, right after Michael formed) for 3 days later. The track is just a little bit off (not bad for a 3-day forecast), and the storm is strong but not quite as strong as the real one (Category 4 in the model, Category 5 in reality).
Now, go back an hour earlier:
Notice the mesovortex (enhanced vorticity) in the SW eyewall? There were winds over 165 mph with that! But then, an hour later, the feature was mostly gone, and the maximum winds were about 140 mph (again, the model forecast was not bad but overall a little weak). I just thought it was interesting that you could see a feature like this in both the real storm and in the model!
This was a pretty nerdy post - I'll probably write a couple more general ones about hurricane forecasting as Hurricane Season approaches :)
Andy
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